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EXTRUSION Molecular orientation occurs when melted polymer is sheared or stretched. If cooled quickly, this orientation is largely retained until the extrudate is reheated to a tem- perature that allows the molecular chains to move to their relaxed state. Orientation can be beneficial or troublesome. It can be desirable when it is deliberately added to the extrudate. For example, it provides property improve- ments such as unidirectional strength in fibers, hoop strength in oriented extrusion blow molded bottles, and tear strength in oriented films. But in many extruded parts orienta- tion can cause post-extrusion problems from non-uniform shrinkage and warpage. Orientation occurs in every extrusion process due to the inherent strain applied to the long-chain molecules as the polymer is shaped and flows through the die. Some orientation through the die is unavoidable, and the magni- tude depends on output rate for a given die design. A secondary annealing process is about the only way to reduce all orientation in extruded parts. Polymer orientation can be measured for transparent polymers in the amorphous state by birefringence, and in both the amorphous and semi- crystalline state by infrared spectroscopy. The simple field test for orientation, often referred to as the "Chrysler test," calls for cutting samples of the extruded part, accurately measuring them, and then subjecting them to heat for a few minutes at a tempera- ture near the processing temperature. After that, the samples are cooled and measured again to determine the shrinkage and/or change in shape. The amount of dimensional change indicates the comparative degree of orientation in that area or direction. In profiles with simple shapes, unbalanced orientation across the extruded cross-section can be reduced by optimizing the die-gap exit to be as dimensionally close to the final shape of the profile as possible to minimize exit velocity differences. Orientation: The Good and the Bad In the case of flat sheet, for example, the die-exit gap should be as uniform as possible across the width. A correctly designed die-flow geometry will result in the same velocity exiting the die across its full width. That's not possible if the die slit is pinched down or opened up somewhere to balance the flow rate. The same is true of annular dies where the die gap has to be adjusted out of concentricity because of unbalanced flow to the die exit. This unbalanced flow of annular shapes occurs frequently in crosshead designs and blow molding dies. More complex extruded shapes with varying cross-sections can result in non-uniform velocity across the profile, resulting in unbalanced orientation that's very difficult to eliminate, requiring post-extrusion heat treating to stabilize the shape. Changes can be made in the die design to balance the velocity and the resulting orientation across the cross-section as the polymer passes through the die-gap exit. The more non-uniform and complicated the geometry, the more difficult this design Get more insights on Extrusion from our expert authors. short.ptonline.com/extrudeKH Learn more at PTonline.com KNOW HOW EXTRUSION In many extruded parts, orientation can cause post-extrusion problems due to non-uniform shrinkage and warpage. Depending on what you are trying to accomplish, molecular orientation can have a positive or negative impact on your part. Here's how to control it. By Jim Frankland All extrusion processes experience molecular orientation due to the inherent strain applied to the long-chain molecules as the polymer is shaped and flows through the die. Die Flow Oriented Alignment Random Alignment Flow 34 AUGUST 2016 Plastics Technology PTonline.com K now How